1. Field of the Invention
The present invention relates to a shading correcting method for correcting a signal A indicative of the image density read by an optical read device on the basis of a reference signal B.
2. Description of the Prior Art
The process of recovering uneven gradation characteristics of a signal read by an optical read device to a correct density level is generally called "shading correction". Such unevenness is caused by photoelectric conversion characteristics which vary according to the image position in a screen of the optical read device. A signal S to be obtained by the shading correction is given by an expression S=A(2.sup.n -1)/B, where A, B and n represent a signal read by the optical read device, a reference signal in a certain region of the screen in which the signal A is obtained, and a bit accuracy for correction, respectively. The operation is performed with respect to the above expression in a manner as shown in FIG. 1. More specifically, when the signal A is, for example, data of three bits (n=3), the signal A is inputted as a column address designation signal into a ROM 11, while the reference signal B consisting of data of three bits is inputted as a row address designation signal. These signals designate an address in which a value obtained by the expression A.multidot.7/B or an approximate value thereof is stored in advance. This value is outputted as a shading correction signal S. Accordingly, as shown in FIG. 2, when the reference signal B equals 7, the signal A is outputted as it is as the correction signal S. When the reference signal B equals 4, 7 is outputted as the correction signal S if the signal A is greater than or equal to 4. On the other hand, if the signal A is 3, 2, 1 or 0 when the reference signal B equals 4, the correction signal to be outputted is 6, 5, 4 or 3, respectively.
In the above-described conventional method, however, the shading correction is performed not by the operation using the signals A and B, but on the basis of a table, which results from the operation and is stored in advance in the ROM 11. A desired value in the table is outputted by designating the column and row addresses using the signals A and B. Accordingly, the conventional method is disadvantageous in that an IC for exclusive use (ASIC), which cannot accommodate any ROM requires an additional external ROM or ROMs. Furthermore, if the operation results to be stored have to be improved in bit accuracy using an IC which can accommodate the ROM, a memory area is required to be doubled for the increase of each one bit, thus causing the ROM to be enlarged in capacity.